1. Field of the Invention
The present invention relates generally to gas turbine engine systems, and more particularly, to methods and apparatus for treating and recovering energy from hot gases, such as primary gas turbine exhaust gases, to improve the overall efficiency of the gas turbine engine, heating or power generating system in which the gases are employed.
2. Prior Art
The use of high pressure gas turbine engines motivated by combustion product gases to provide mechanical shaft energy, which is convertible into electrical energy, is common in modern day power plants. In recent years, due in part to increasing fuel costs, there has been an especially intensive effort by designers to increase the thermodynamic efficiency of the gas turbine. It is generally accepted that the efficiency of such a turbine is a function of the turbine operating temperature (T.sub.o) and turbine exhaust gas temperature (T.sub.x) and that the efficiency relationship may be expressed as: EQU efficiency=(T.sub.o -T.sub.x)/T.sub.o
Thus, in a typical situation, where T.sub.o =1,073.degree. K. and T.sub.1 =773.degree. K., the turbine efficiency is only 27.8%. Most efforts to improve turbine efficiency have focused on increasing T.sub.o, the turbine operating temperature. However, increased temperatures necessitate the use of improved materials, which are generally more costly, while the exposure to higher temperatures shortens the lifetime of the equipment. Thus, on an overall economic basis, it is doubtful that merely increasing T.sub.o is a productive approach to improving turbine efficiency.
Designers have had very little success in lowering T.sub.x, the turbine exhaust gas temperature. As a result, the relatively low pressure, relatively high thermal content turbine exhaust gases are typically used as the thermal source in conventional heat exchangers and passed in indirect heat exchange relationship with water or compressed air to produce steam or combustion chamber feed air before being disposed of by exhausting them to atmosphere through tall chimneys or stacks. It has also been suggested that overall turbine system efficiency could be increased by conducting the relatively high thermal content exhaust gases to a low pressure turbine to produce additional mechanical shaft work before passing the resulting exhaust gas, in typical manner, through conventional heat exchangers to indirectly heat water or compressed air before discharging the gases to atmosphere through tall chimneys or stacks.
The problem with either of the foregoing approaches to utilizing the exhaust gases is that they typically contain harmful contaminants yet must be discharged through stacks to the atmosphere. The disadvantages of this method of disposal include resulting air pollution and its harmful effects on the environment, a waste of recoverable heat energy, and the high cost of constructing and maintaining tall stacks. Loss of recoverable heat energy is unavoidable because gases discharged into a stack must be substantially hotter than ambient air to produce an up-draft in the stack and to avoid condensation in the chimney. Moreover, the latent heat of steam in flue gases is not generally recovered in order to avoid condensation and the attendant corrosion, as a result of which additional, available heat energy is being wasted.
Where the latent heat of steam is not recovered, the system designer must work with "low heating values" of the fuels rather than "high heating values". Low and high heating values for fuels are given in such handbooks as the John N. Perry Engineering Manual, published in 1959 by McGraw Hill, where the following typical heating values are given:
______________________________________ High Heating Low Heating Gas Value Value ______________________________________ Hydrogen 60,958 Btu/lb 51,571 Btu/lb Methane 23,861 Btu/lb 21,502 Btu/lb Octane 20,510 Btu/lb 19,150 Btu/lb Methyl 10,270 Btu/lb 9,080 Btu/lb alcohol (vapor) ______________________________________
As will be apparent from these heating values, about 18 percent more Btu/lb can be recovered from hydrogen if its high heating value can be utilized, about 11 percent more from methane, about 13 percent more from methyl alcohol vapor and about 7 percent more from octane "gasoline". Prior systems have not been able to utilize the high heating value of such gases.
As the public concern about air pollution has increased, stack heights have been increased to affect better dispersion of pollutants. However, increasing stack heights adds to the cost of constructing and maintaining stacks, yet provides no solution to the underlying problem, i.e., avoiding emission in the first instance of harmful substances such as sulfur oxides, chlorine gases, phosphor oxides, etc.
A significant factor in air pollution is the increasing level of gaseous airborne pollutants which combine with moisture in the air to produce acids, e.g., carbon dioxide, sulfur dioxide, and compounds of chlorine and fluorine. The carbon dioxide content is some industrial districts is as high as ten times normal. Acid forming pollutants have been found in some instances to increase the acidity of rainwater from its normal pH of about 6.9 to values of 4.0. Rainwater having a pH of 5.5 or lower less will destroy aquatic life and can do substantial harm to buildings, monuments, and other structures.
One proposal for removing acid forming components from exhaust gases is to scrub the entire flow of exhaust gases with water and caustic prior to discharging them through a stack. However, scrubbing the entire exhaust gas flow requires large quantities of water, which are not always available, and requires costly, large capacity scrubbing equipment. Indeed, scrubbing the entire flow of exhaust gases from some incinerators requires at least half the amount of water, by weight, of the solid wastes burned in the incinerator. Treating the large volume of scrub water needed in such a process is very costly and contributes to the impracticality of scrubbing as a total solution to the acid pollutant problem.
Another difficult pollutant to deal with effectively is sulfur in the exhaust gases. One proposal for the desulfurization of exhaust gas utilizes a series of heat exchangers to extract heat energy from the gas prior to a scrubbing operation. Heat extracted from the gas is returned to the gas following desulfurization and the gas is exhausted through a tall stack for diffusion into the atmosphere. This proposal has the disadvantages of wasting heat energy recovered from the gases, requiring large volumes of scrubbing water, requiring the use of a tall stack, and polluting the air with such noxious components as are not removed during scrubbing.
The problem of disposing of exhaust gases is now recognized as a major concern in industrial countries throughout the world. Dispersing emissions through the use of tall stacks is no longer regarded as an acceptable solution. Applicant's U.S. Pat. No. 3,970,524 discloses a system for gasification of solid waste materials and a method for treating the resulting gases to produce commercially useable gases in such a manner that dispersion through stacks is not necessary. A feature of one embodiment of this patent is pressurization of a combustion zone to such pressures as will permit blower and/or compression units to be eliminated from the gas treatment system. Another feature is the use of a multichamber gas treatment unit in which noxious gas components are sublimed or "frozen out" and thereby separated from the clean useable gas components. A problem not addressed by U.S. Pat. No. 3,970,524 is that of providing a system for treating combustion exhaust gases and productively reclaiming heat energy from the hot gases. This problem is, however, dealt with in applicant's U.S. Pat. No. 4,126,000 which teaches reclamation of heat energy by the transfer of the sensible and latent heat of the gases to a power fluid in indirect heat exchange relationship therewith, as in a conventional heat exchanger. However, the economics of indirect heat exchange at the lower temperature levels are very poor and reduce the overall desirability of such a system. Applicant's U.S. Pat. No. 4,265,088 (copending Application Ser. No. 962,103, filed Nov. 17, 1978,) discloses a system which utilizes direct heat exchange between the hot gases and a power fluid to improve the economics and thermal efficiency of the system.